LECT. VII.] ANATOMY OF STEMS. 337 



be unusually cold, or if the leaves of the tree or 

 the shrub happen to be much devoured by caterpil- 

 lars, it gains very little that season in diameter. 

 From the same cause the zones are also of un- 

 equal degrees of hardness : but, independent of 

 the comparative density of each, the hardness of 

 the whole increases with the age of the tree, so 

 that they are hardest in the centre, and less and 

 less hard as they approach the circumference. 

 The outermost layer, being alburnum, is always 

 soft, and continues so until another layer is formed 

 over it ; but if the tree be barked the alburnum 

 assumes the apparent character of wood in the 

 same year ; and hence it has been recommended 

 to bark trees the year before they are intended to 

 be cut down. "The German foresters," how- 

 ever, " have proved that wood treated in this 

 " manner is less elastic, and is more easily in- 

 " jured by humidity and insects * ;" which I 

 conceive is owing to the natural change of al- 

 burnum into wood not depending on a simple 

 hardening or condensation ; but on such a de- 

 position of ligneous and other particles in its tex- 

 ture, as tends to increase the cohesive attraction 

 of all its parts, and consequently to augment 

 both its hardness and its elasticity ; while the ex- 

 posure of the alburnum, by stripping off the bark, 



* Mirbel, Elemens de Phys. veg, l cfe partie, p. 110. 

 VOL. 1. Z 



